This invention relates to a vessel speed detecting device and more particularly to an improved indicator arrangement for such devices.
A variety of devices have been proposed for use in indicating the speed of travel of a watercraft through the body of water in which it is operating. Such devices do not provide high degrees of accuracy under all conditions because of the many uncontrollable variables present in a watercraft. However, a type of watercraft speed indicating device that has been found to be very accurate over a wide range of watercraft speeds is disclosed in the copending application for U.S. Letters Patent entitled "Speedometer For Marine Vessels", Ser. No. 160,124, filed Feb. 25, 1988 in the name of Kazuhiro Nakahama et al, and assigned to the assignee of this application. In that type of device, there is provided an opening in the hull or some component associated with the hull against which dynamic water pressure acts. This water pressure is transmitted to a semiconductor pressure device that provides an output signal indicative of pressure and which can be related to watercraft speeds. Although this type of device has particular utility, there are certain disadvantages in that under low speeds, the accuracy or resolution of the device is not as good as could be desired. This is caused because the following equation expresses the relationship between dynamic pressure (P) and the watercraft speed (V): ##EQU1## Thus, as the watercraft speed is reduced, the ratio of watercraft speed to the increment of dynamic pressure varies along a curve having a configuration as best shown in FIG. 1.
However, this type of device generally cooperates with an analog to digital converter (ADC) that digitizes the output of the semiconductor pressure sensor by dividing it uniformly into a number of steps, such as 256 steps. Therefore, as the vessel speed becomes lower, the resolution of the ADC and detecting accuracy deteriorates. Furthermore, the low pressure level to be detected further gives rise to difficulties in correct speed indication.
Also, at low speeds, the dynamic pressure is unstable because of the response time lag caused by the inertia of the flowing water and the effect of temperature changes or so on can give rise to unreliable speed indication. Therefore, it is not an uncommon practice to calibrate the analog speed indicator so that speeds below a certain speed such as 20 kilometers per hour are not registered as shown by the indicator dial of FIG. 2.
It is, therefore, a principal object of this invention to provide an improved vessel speed detecting device and an indicating system therefor.
It is a further object of this invention to provide a vessel speed detecting device that indicates when the watercraft speed is below a speed at which the accuracy of the system is as good as desired.